I was trying to find a way to reference the 'APP01' part of http://xxx.adobe-campaign.com/webApp/APP01 within the webapp so that I don't have to hard-code the URL in the HTML. Is this achievable?
One possible solution.
You can create an option and store the internal name of web app there and fetch it in webapp using the following code.
var intName = getOption('optionInternalName');ctx.vars.webappInternalName = intName;
If it's client-side js can just get it from (window.location + '').replace(/.*\//, '').
Server-side doesn't have any info about the webapp afaik.